The present invention generally relates to flexible printed circuit boards, and in particular to the mounting of components thereto.
For mounting electrical and other components to a flexible printed circuit boards according to prior art, the components are positioned at the appropriate locations and subsequently fastened by gluing or soldering. There is often a problem to maintain the components at the appropriate positions with a sufficient precision, during the gluing or soldering process.
The European patent application EP 0 256 581 A1 discloses a method for fixation of electric components at a flexible printed circuit board, which is intended to hold the component during the soldering or gluing process. This method is based on that tabs are formed in the flexible printed circuit board, which tabs extend into an opening in the flexible printed circuit board, large enough to allow the component to pass through. By introducing components from the back side of the board, the tabs are bent and applies a force onto surfaces of the component which are perpendicular to the board surface. The tabs clamp the component between each other and by prohibit the component to move back again, the component is fixed. The method is intended to be ended by a soldering or gluing process, which finally fastens the component. The tabs may also be covered with a conducting layer, which provides a possibility to achieve electrical contact by soldering or gluing with a conducting adhesive.
Most electric circuit mounting processes of prior art relies on soldering or gluing for achieving a good electrical contact and for maintaining the component mechanically in a fixed position. However, gluing or soldering processes are not always desired, in particular for very small components. There is always a risk for not achieving a sufficient reliable joint or to achieve a shortening between different electrical parts. Furthermore, the soldering and gluing occupies a separate production step, which increases the mounting times. If a component is found to be defect after mounting, there are also large problems to replace the defect component with a new one. When soldering, the joint may be reheated to melt the soldering, but for gluing, the normal procedure is to reject the total board.
Many printed circuit boards also uses separate assembling structures to hold the different components. The European patent application EP 0 649 272 A1 discloses a flexible fastening member which is used to reduce stress caused by differing temperature coefficients for the board and the components. However, a fastening member of that type increases the complexity of the assembling procedure considerably.
There is also a general problem in finding cheap and simple solutions on contacting members. One solution is disclosed in the U.S. Pat. No. 5,049,089. In this document a method is described, which uses a portion of a flexible printed circuit board provided with electrical leads, which portion is bent and stuck into opening in another circuit board. The elastic force keeps the portion in position when applying the soldering material.
The particular solution, mentioned in EP 0 256 581, has the further problems of mechanically holding the component only by the tabs, why the heat exchange with the printed circuit board is very low, with overheating as a problem. Furthermore, the total mass of the component is mechanically fixed only by the limited tabs, which easily may cause fractures in the soldering.
The overall object of the present invention is to achieve a method of electrical circuit board assembling, which may omit the soldering or gluing step. Another object of the present invention is to reduce the number of separate parts used for assembling and electrically connecting. A further object of the present invention is to mount the components at the printed circuit board in a removable manner, for simplifying replacement of components or function as an electric connector.
The above objects are provided by an arrangement and mounting method according to the enclosed claims. In short terms, the present invention can be described as using the resilient behaviour of the flexible printed circuit board both to mechanically clamp the components in a permanent manner and to achieve good electrical contacts between conducting parts of the flexible circuit boards and the components. By cutting the flexible printed circuit board in such a manner that small tab-like, resilient members are formed, the forces caused by elastically deformed resilient members are usable both for mechanically fixing the components and for causing an electrical contact. By choosing appropriate sizes of the resilient members, the relative strength of the spring force is increased, which even will be large enough to cause a plastic deformation of the material in the contact points between the conducting resilient members and the component contact members. In such a manner, soldering or gluing may be totally omitted.